1. Field of the Invention
The present invention relates to porous orthopedic implants. More particularly, the present invention relates to porous orthopedic implants made from polymeric preforms, and to a method of manufacturing the same.
2. Description of the Related Art
Orthopedic implants may be constructed of porous biomaterial to encourage bone growth into the orthopedic implant. An example of such a material is produced using Trabecular Metal™ technology generally available from Zimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is a trademark of Zimmer, Inc. Such a material, which is also referred to as “TM” in the following, may be formed from a reticulated vitreous carbon (RVC) foam substrate which is infiltrated and coated with a biocompatible metal in the manner disclosed in detail in U.S. Pat. No. 5,282,861 to Kaplan, the disclosure of which is expressly incorporated herein by reference. The resulting, coated material is lightweight, strong, and has open cells that resemble the structure of natural cancellous bone, thereby providing a matrix into which cancellous bone may grow to fix the orthopedic implant to the patient's bone.
The starting material in this process is an open-cell polymer foam block or sheet. Polymer foams may be made by the controlled expansion of gas during the polymerization process. Standard polyurethane, for example, is made by reacting polyisocyanates and polyols. Polyurethane foam, on the other hand, is typically made by reacting polyisocyanates, polyols, and additionally a blowing agent, such as water. During the polymerization process, the water reacts to form carbon dioxide gas that expands and escapes from the surrounding polyurethane, leaving behind open cells surrounded by polyurethane ligaments.
The polymer foam starting material is then converted into an RVC foam substrate. This step may involve first impregnating the polymer foam with a carbonaceous resin and then heating the impregnated foam to a suitable pyrolysis temperature, on the order of 800° C. to 2000° C., to convert the polymer foam and any carbonaceous resin into vitreous carbon. This process is described in U.S. Pat. No. 6,103,149 to Stankiewicz, the disclosure of which is expressly incorporated herein by reference. The RVC foam substrate is then infiltrated and coated with a biocompatible metal, as discussed above.
Using an open-cell polymer foam as the starting material for a porous orthopedic implant presents certain challenges.
First, achieving the desired, final shape of the orthopedic implant may be difficult, because polymer foams are typically provided in blocks or sheets that must be machined at some point in the process into the desired, final shape. For example, the component may be machined while in the polymer foam form, the RVC foam form, or the coated metal form. Such machining is both expensive and time-consuming. Also, such machining may damage the ligaments that define the open-cell pores of the foam, especially when the component is shaped while in the brittle, RVC foam form. Finally, such machining is wasteful, because scraps from the bulk blocks or sheets may have to be discarded.
Second, the minimum thickness of the porous orthopedic implant is limited. In general, the porous orthopedic implant must be at least as thick as several supporting polymer ligaments, which may be about 0.5 mm in length each. Therefore, it can be difficult to manufacture small implants, such as dental implants and orthopedic fasteners, with a process based on polymer foam.
Third, polymer foam is entirely porous. Therefore, polymer foam does not provide non-porous regions, which may be desirable in an orthopedic implant.